Selective etching process for changing shadow-mask aperture size

ABSTRACT

The shadow mask of a color picture tube is formed by etching a blank to provide a field of apertures individually having a large diameter portion and a coaxially aligned, small diameter portion attached to the former by a thin wall section. In screening a tube which is to utilize such a shadow mask for color selection, the phosphor materials are deposited on the screen in a photographic process involving the exposure of a layer of photosensitive material by actinic energy directed through the small diameter portions of the mask apertures. After screening has been accomplished, the thin wall sections are etched away, leaving the mask with apertures of large diameter, larger than the phosphor deposits in the screen.

O Unlted States Patent [151 3,653,900 Black 1451 Apr. 4, 1972 54] SELECTIVE ETCHING PROCESS FOR 2,961,313 11/1960 Amdursky ..96/36.l CHANGING SHADOW.MASK 3,231,380 l/l966 Law ..96/36.l APERTURE SIZE Primary Examiner-Norman G. Torchin [72] Inventor: Joseph M. Black, Chicago, ill. Assistant Examiner-John Winkelman At: -J h L? d [73] Assignee: Zenith Radio Corporation, Chicago, ill. omey n e erson.

[22] Filed: Aug. 15, 1969 ABSTRACT [21] APPL 350 408 The shadow mask of a color picture tube is formed by etching a blank to provide a field of apertures individually having a large diameter portion and a coaxially aligned, small diameter [52} U.S.Cl. ..96/36.1, l i7/33.5 CM,3l3/85, portion attached to the former by a thin wall section. In 313/92 B, 156/7 screening a tube which is to utilize such a shadow mask for [51] Int. Cl ..G03c 5/00 color l n. the p ph r materials r deposited on the 58 Field of Search ..96/36.l, 36.2; 156/2, 7; screen in a p g p process n ing th ex osur of a 117 335 313 35 layer of photosensitive material by actinic energy directed through the small diameter portions of the mask apertures. [56] References Cited After screening has been accomplished, the thin wall sections are etched away, leaving the mask with apertures of large UNITED STATES PATENTS v diameter, larger than the phosphor deposits in the screen.

2,750,524 6/1956 Braham ..96/36.l 6 Claims, 5 Drawing Figures h lld t BACKGROUND OF THE INVENTION One of the most attractive developments in the shadowmask color television tube is the black surround screen described and claimed in U. S. Pat. No. 3,146,368, issued on Aug. 25, 1964, and assigned to the assignee of the present invention. The principle of black surround is applicable whether the phosphor deposits comprising the screen are in the form of strips, hexagons or dots. The configuration of the deposit is of no particular consequence but, for convenience, attention will be directed initially to the dot triad screen structure having black surround. Such a screen differs from antecedent devices in that the phosphor triads have dots of various phosphors which are smaller in diameter than the apertures of the shadow mask, whereas in earlier devices the phosphor dots of the triads are larger in diameter compared to the apertures of the mask and are in essentially tangential contact with one another. By reducing the diameter of the phosphor dots, there is provided a separation between the dots to which may be applied a light-absorbing material or pigment. Such a material is usually black and surrounds the phosphor dots from whence its name derives.

It will be apparent on reflection that such a tube presents problems in screening; in particular, it imposes the condition that the phosphor deposits be smaller in dimension than the apertures or electron transparent portions of the shadow mask which is also referred to as the color-selection electrode.

Screening a tri-color tube is most easily accomplished by photographic techniques in which the image area of the tube is covered with a light sensitive material that is to be exposed by actinic energy directed to the screen through the apertures of the shadow mask. Such an approach is beneficial in that it accomplishes precision in the positioning of the phosphor deposits on the image area of the tube in relation to the apertures of the shadow mask. Generally, the mask is in such juxtaposition to the screen that the phosphor deposits, determined through such an exposure, are about the same in configuration but larger in dimension than the apertures of the mask. Therefore, some deviation from normal screening practices is required to satisfy the condition of the black surround tube that its phosphor deposits be smaller in size than the mask apertures.

A variety of approaches to this problem have been suggested, representative ones being the subject of U. S. Pat. No. 3,070,441, issued on Dec. 25, 1962 and U. S. Pat. No. 3,23 l ,3 80, issued on Jan. 25, 1966. These earlier efforts contemplate that the mask will be formed in conventional fashion with apertures of normal size. The apertures are then stepped down or reduced in size on a temporary basis for the purpose of having proper dimensions for screening. To that end, the holes are temporarily filled with a material, usually a metal different from that of the mask blank, which is removed after screening has taken place. The filler may be removed, for example, by etching. In theory, this is an acceptable solution but it raises practical problems because the filler is generally applied while the mask is still in sheet or planar form and when it is pressed into its required dome shape, difficulties of cracking or distortion are frequently experienced. Similar problems,-

giving rise to imperfections in the phosphor deposits, can be expected in most processes in which the mask is formed with large size apertures that are to be stepped down by some temporary scheme of closing or filling the apertures. The present invention is a different solution to this problem which avoids difficulties of the earlier art and permits better control and uniformity of both the mask apertures and phosphor deposits.

Accordingly, it is an object of the invention to provide a novel shadow mask for a color picture tube as well as a unique process for screening such a tube.

It is another and particular object of the invention to provide an improved process for screening a shadow-mask type of color picture tube featuring black surround.

It is another object of the invention to provide an improved shadow mask to be used for the fabrication, and in the operation, of a shadow mask type of color picture tube.

SUMMARY OF THE INVENTION A shadow mask for a shadowmask type of color picture tube embodying the invention comprises a metallic sheet having a field of apertures which individually have a first portion of a given configuration (a slot or circular hole depending on the shape desired for the phosphor deposits) and a second but smaller portion of the same configuration. The second portion is disposed in nested relation to the first portion and is attached thereto by a wall section having a thickness which is but a small fractional portion of the thickness of the sheet. For the dot triad type of tube the aperture portions are circular and the interconnecting wall section is provided by an annular recess that is concentric with the small diameter portion of each mask aperture.

In the process of screening a shadow-mask tube, the mask as described in the preceding paragraph is used in the photographic deposition of the various phosphor materials and after these materials have been applied to the screen or image area of the tube, the interconnecting wall sections of the mask are etched out. This removes the smaller portion of each aperture of the mask, leaving only the larger portion thereof and presenting a shadowmask which is essentially conventional in configuration and dimensions.-

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a fragmentary view of the screen section of a shadow-mask type of color picture tube and also of a shadow mask in screening relation to the image area of the tube;

FIG. 2 is a view on an enlarged scale of one of the apertures of the mask represented in FIG. 1;

FIGS. 3 and 4 are views, likewise on an enlarged scale, of another form of aperture that may be provided in the shadow mask; and

FIG. 5 representsstill another form of mask or color-selection electrode embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Color tubes of the type under consideration may have circular or rectangular envelope configurations at the faceplate or screen section which defines the image area. in this area, there is applied a repeating sequence of phosphors representing the three primary colors because commercial television as presently practiced is an additive type of system. The phosphor deposits may be strips, hexagons, circular dots or the like, as stated above. The specific configuration of the envelope or of the elemental phosphor deposits is of no particular consequence to the present invention but for convenience it will be assumed that the tube under process is a rectangular one having a mosaic screen defined by a multiplicity of dot triads with each such triad including a dot of green phosphor, a dot of blue and a dot of red phosphor. In the fragmentary view of FIG. 1, the circles denote phosphor deposits and the legends G, B and R symbolically represent those of the deposits which are formed of green, blue and red phosphors, respectively. It will be observed that the phosphor dots are smaller than usual and, therefore, are not in tangential contact; instead, they are separated from one another and the area, of the screen that surrounds each such dot may accommodate a light-absorbing material in accordance with the teachings of U. S. Pat. No. 3,146,368, although for convenience of illustration, that pigment has not been represented in the drawing. Moreover, a shadow mask tube featuring postdeflection-focus employs the same screen arrangement whether or not it also has black surround.

Superposed over and in close spaced relation to the screen or image area of the tube is a shadow mask 10, only a portion of which is shown in FIG. 1. It is a relatively thin metallic sheet, usually formed from cold rolled steel having a thickness of approximately 5 mils. This sheet has a multiplicity of apertures 11 arranged in a field with dimensions and configuration corresponding to the dimensions and configuration of the image area. As shown in the enlarged view of FIG. 2, each individual aperture of the mask has a large diameter portion 1 la of generally conical shape having a maximum diameter D Each aperture of the field also has a coaxially aligned, small diameter portion 1 1b of much the same shape and with a maximum diameter D,. The dimension D represents the part in common to portions 11a and 1112 or the area over which the one opening is in communication with the other. The smaller diameter portion 1 1b is attached to large diameter portion 1 1a by a wall section 11d having a thickness t that is a small fractional portion of the thickness of the sheet from which the mask has been made. So long as the apertures of the mask are constructed as described and as illustrated in FIG. 2, they present an effective aperture having a dimension D chosen to the end that phosphor dots applied to the image area of the screen in a photographic technique, considered hereinafter, are of a desired size.

Before considering the screening process, as such, it is appropriate to describe a method of forming a shadow mask having apertures of the type illustrated. It is most convenient to use a metal stock, such as cold rolled steel, into which apertures may be formed, as by etching, with precision not only as to their location within a desired field but also as to dimension. Procedures for accurately and controllably etching apertures in such a metal sheet are totally within the skill and knowledge of those engaged in etching of sheet metal. The general process, for example, is described in the following U. S. Pat. Nos. 2,750,524 issued on June 12, 1956; 2,762,149 issued on Sept. 11, 1956; and 2,961,313 issued on Nov. 22, 1960.

By and large, the process involves well-known techniques of photoprinting by which a pattern is established on a sheet followed by chemical milling or etching through which the pattern is developed. More particularly, in considering the aperture structure of FIG. 2, the mask blank is first coated on both sides or surfaces with a photosensitive material having such properties that its solubility in a given solvent is influenced by exposure to actinic energy. Preferably, an organic photosensitive material is employed of such nature that those portions thereof that are exposed to ultraviolet light are rendered insoluble. Having applied coatings of this material to both surfaces of the blank, a design is projected on each face thereof by ultraviolet light to transfer that design by the exposure. Specifically, the pattern of the design for the mask under consideration shields each aperture portion 11a on one surface of the mask blank from exposure and similarly shields each portion 11b on the obverse side of the blank. The design projected on the same surface of the sheet as the smaller aperture portion 11b is further arranged to shield from exposure a concentric annulus or ring 11c having an inner diameter which is less than the diameter D of aperture portion 1 la and an outer diameter substantially equal to the dimension D Of course, the designs are projected from accurately determined positions so that shielded portions of the photosensitive coating representing aperture portions 11a and 11b are in coaxial alignment throughout the aperture field. Aside from these shielded portions associated with each aperture to be formed in the mask blank, the entirety of the blank is exposed on both surfaces and the image resulting from such exposure is developed by spraying, washing or otherwise treating the surfaces of the blank with the solvent for the photosensitive coating. In this treatment, the exposed portions of the photosensitive material remain in situ while the rest of the coating is washed off. As a consequence, the surface area of the blank at the location of each aperture portion that is to be formed as well as at the situs of each concentric ring 11c is stripped of its coating and is thus exposed or laid bare. These exposed elemental areas of the mask blank are then subjected to an etch bath of ferric chloride of suitable concentration. While the exposed elemental areas of each side of the mask blank may be etched individually, it is preferred, as described in certain of the afore-identified patents, to etch both sides or faces of the mask blank simultaneously.

The portion designated D which is common to aperture portions 1 la and 1 lb is determined or controlled as to size by the diameters D and D, and the etching time. Concurrently with etching out aperture portions Ila, and 11b, the thin wall section 11a is formed by removing most of the transverse section of the mask blank intervening portions 11a and 11b so that wall section 11d may, at a subsequent time, be removed with facility, speed and accurate control. In etching out the recess 11c to form wall section 11d, the inner and outer diameters defining annulus 11.-- are dimensioned so that the annular recess has a depth h that is at least equal to, but preferably exceeds, the depth h of the small diameter portion 11b. Controlling the depth of this recess determines the thickness t of the interconnecting wall section. As stated, it is made as thin as possible while preserving sufficient mechanical strength that the mask blank, having been provided with the described field of apertures, may be domed or formed without destroying or breaking free the small aperture portions 11b. A thickness of about l-2 mil is believed to be acceptable.

The shadow mask with the field of formed apertures is appropriate for use in screening the picture tube. In the first screening step, if the tube is of the dark surround variety, the image area of the tube is covered with a photosensitive resist composition in which light-absorbing material, such as manganese carbonate, is carried in suspension as described in U. S. Pat. No. 3,365,292. This resist is of the type that it is rendered soluble upon exposure to actinic energy and the coated screen is exposed three times with each exposure being made through the shadow mask. In this process step, actinic energy, such as ultraviolet light, is able to reach the screen area only through aperture portions 11b so that each exposed area of the sensitized coating is a circle of dimension D,. In each of the three exposure steps, the light source is positioned to simulate one of the three electron guns of the tube and in this fashion, latent images are established of the elemental areas of the screen that are to receive the phosphor deposits. Washing the exposed screen with a solvent for the photosensitive resist develops these images by removing all exposed portions of the coating. What remains on the screen is the black surround material as a suspension in an organic material that will be removed in bakeout.

The screen is now in condition to receive the phosphor deposits and each material is applied by means of a similar photographic technique in which the image area is first covered with a layer of photosensitive resist material carrying one of the three phosphors in suspension. In this instance, however, an opposite type resist is employed. That is to say, the resist is of the kind that is rendered insoluble by exposure to ultraviolet light. Polyvinyl alcohol sensitized by ammonium dichromate is a very useful resist for this purpose since it is soluble in water. After the exposure, latent images of the deposits of a particular phosphor will have been made and they are developed by washing the screen with a solvent, such as water, leaving deposits of the phosphor properly located on the screen area relative to the black-surround material. In like manner, the remaining two phosphor materials are applied but in each instance, the location of the exposing light source is adjusted to simulate the assigned beam of the tube in process. At this juncture, the screen will have been completed so far as the deposits of phosphor and the black-surround material are concerned. It may now be aluminized in the usual way so that the phosphor dots have a backing layer of an electron permeable, conductive and light reflecting metal, usually aluminum.

After aluminizing, the screen is ready for bakeout although this step is usually taken with the mask in position within the faceplate section of the tube.

With the screening accomplished, the small diameter portion 11b of each aperture of the mask may be removed and this is achieved by a second etching process in which the wall sections 11d are etched out. Having removed aperture portion 11b, each aperture of the mask will have been enlarged to the dimension D which is controlled by the etching step utilized to remove wall sections 11d. The mask is now conventional, having apertures of normal size, and is suitable for securing into the faceplate section of the tube in essentially spaced parallel relation to the screen area, preparatory to bake out.

A shadow mask constructed in accordance with the invention has distinct advantages in fabricating a color tube. Frequently, the mask is made by a fabricator having equipment with suitable controls to accurately position and dimension aperture portions 11a, 11b and recess 11c. The accuracy attainable, particularly as to aperture portion 11b, provides the desired position, configuration and dimension of the phosphor dots deposited on the screen of the tube by exposing through such a mask. Distortions of the spot configuration and nonuniformities attributable to past techniques of temporarily filling in the apertures of the shadow mask are avoided. Another advantage is the simplicity with which a tube manu facturer, having used the mask for screening purposes, may remove aperture portions 11b and convert the structure to a mask with holes that are larger than the phosphor deposits as required for a black-surround or post-defiection-focus type of picture tube. Since the wall sections t are very thin, they may be etched away in a very short period of time, in a few minutes, if desired, and without requiring the sophistication of control desired in originally etching and forming mask portions 11a, lllb and 110. Still another advantage results from the fact that the second or re-etch, being accomplished in a short period of time, removes a minimal amount of the surface of the mask which intervenes each of its apertures. This is desirable because otherwise it would be necessary materially to increase the thickness dimension of the mask blank with attendant disadvantages in processing times and material costs.

If desired, the modified aperture structure of FIGS. 3 and 4 may be employed. In this case, large diameter portions 11a and Ila are formed in opposed elemental areas on both surfaces of the blank leaving, partway in the thickness of the blank, the small diameter portion 11b. In this modification and as shown in FIG. 3, the thin interconnecting wall sections 11d may be formed by etching out arcuate sections 1 1e distributed about a path concentric with aperture portion 11b. If sections 112 are etched through, it is desirable to fill them with an opaque material so that they do not occasion unwanted exposures during the screening process. Alternatively, these portions may be partially etched out, leaving a wall section 11s so that only the part 11b of each aperture is transparent to actinic energy.

Of course, the multiplicity of apertures in the shadow mask may be identically dimensioned or, if desired, they may be weighted. For example, it is sometimes arranged that the diameter of the apertures are largest at the center of the mask and decrease with radial spacing from the center in order to obtain a brightness increase at the center of the screen and to provide an acceptable tolerance for beam registration at the edges of the screen. This result may be realized in the practice of the present invention simply by photoprinting with an aperture pattern that has the hole weighting desired.

As indicated above, the specific configuration desired of the aperture in the mask is of no moment in practicing the invention and the enlarged fragmentary view of FIG. 5 shows the application of the invention to a mask having elongated slots rather than circular holes. More specifically, each opening in the mask has a first portion 21a in the form of a slot of width D extending entirely across the mask blank except for its ends (one of which appears in FIG. 5). The ends are imperforate and border the series of slots 21a. Each opening also has a second but smaller portion 21b, likewise in the form of a slot of width D disposed on the opposite side of the mask blank in nested relation to portion 210 and attached thereto by narrow wall sections 21d formed by recesses 21c etched out of the blank on opposite sides of each slot 2112.

In using this mask, a line or strip type screen is applied photographically with the location and dimensions of each phosphor deposit determined by actinic energy projected onto the screen surface through slots 211b. After screening, the mask is re-etched to sever wall sections 21d and cause the screen to have only the large slot-type apertures 21a.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The process of preparing the screen area of a shadow mask type of color picture tube which comprises:

selectively etching one surface of a metallic mask blank to form a first multiplicity of aperture portions of a given configuration; selectively etching the opposite surface of said mask blank to form a second and corresponding multiplicity of aperture portions of substantially the same configuration but smaller in size than and disposed in transverse alignment with said first multiplicity of aperture portions and further to form in said mask blank a multiplicity of channels individually surrounding each of the aperture portions of said second multiplicity to provide wall sections having a thickness that is a small percentage of the thickness of said mask blank and attaching the aperture portions of said first and second multiplicities to one another, said apertures being disposed in a pattern related to the pattern of phosphor distribution desired for said screen area of said tube;

depositing on said screen area of said tube a plurality of different phosphor materials individually applied by means of a photographic technique in which said screen area is first covered by a layer of photosensitive material and is then exposed through said second multiplicity of aperture portions with actinic energy to determine the location on said screen area of deposits of the phosphor material being applied;

etching out said wall sections of said mask to remove said second multiplicity of aperture portions;

and thereafter securing said mask in spaced essentially parallel relation to said screen area of said tube.

2. The process in accordance with claim 1 in which said wall section is formed by etching out a major portion,

but less than all, of the transverse section of the mask blank material which intervenes said first and said second multiplicities of aperture portions.

3. The process in accordance with claim 1 in which said first and second multiplicities of aperture portions are circular and in which said wall section is formed by etching out an annulus having an inner diameter which is less than the outer diameter of the aperture portions of said first multiplicity but is greater than the outer diameter of the aperture portions of said second multiplicity;

and in which the depth of said annulus is substantially equal to that of the aperture portions of said second multiplicity.

4. The process in accordance with claim 3 in which said annulus has a depth exceeding that of the aperture portions of said second multiplicity.

5. The process in accordance with claim 3 in which the outer diameter of said annulus is substantially equal to the diameter of the aperture portions of said first multiplicity.

6. The process in accordance with claim 1 in which said mask blank is selectively etched to have the thickness dimension of the aperture portions of said first multiplicity substantially greater than that of the aperture portions of said second multiplicity. 

2. The process in accordance with claim 1 in which said wall section is formed by etching out a major portion, but less than all, of the transverse section of the mask blank material which intervenes said first and said second multiplicities of aperture portions.
 3. The process in accordance with claim 1 in which said first and second multiplicities of aperture portions are circular and in which said wall section is formed by etching out an annulus having an inner diameter which is less than the outer diameter of the aperture portions of said first multiplicity but is greater than the outer diameter of the aperture portions of said second multiplicity; and in which the depth of said annulus is substantially equal to that of the aperture portions of said second multiplicity.
 4. The process in accordance with claim 3 in which said annulus has a depth exceeding that of the aperture portions of said second multiplicity.
 5. The process in accordance with claim 3 in which the outer diameter of said annulus is substantially equal to the diameter of the aperture portions of said first multiplicity.
 6. The process in accordance with claim 1 in which said mask blank is selectively etched to have the thickness dimension of the aperture portions of said first multiplicity substantially greater than that of the aperture portions of said second multiplicity. 